Insecticides have enjoyed widespread use in commercial agriculture, and have enabled an enormous increase in crop yields and product quality. Pesticides are also routinely used to control various insects, as for example flies or mosquitoes, when pest populations pose a nuisance or health hazard to humans or livestock. There is, however, an increasing awareness of environmental risks associated with the use of certain synthetic pesticides, including concern over the bioaccumulation of pesticides in the food chain or their detrimental effects on non-target organisms. Biological pesticides and especially natural biopesticides have therefore been of considerable interest to those seeking environmentally acceptable means of pest control.
The microorganism B. thuringiensis has long been recognized to be useful in the control of insect pests. The sporulating B. thuringiensis cell produces a class of compounds, formerly regarded as a single .delta.-endotoxin but now understood to comprise several distinct toxin proteins, which are concentrated in a crystalline protein inclusion body found in the endospore. Upon ingestion of the inclusion body by a susceptible insect larva and proteolysis in the insect gut, the endotoxin proteins are converted into active compounds which destroy the gut epithelium and ultimately the pest itself.
B. thuringiensis .delta.-endotoxins have accordingly been found to be useful as pesticides when applied in the form of lysates or other fermentation extracts of cultures of the microorganism. These toxins show remarkable activity against a variety of Lepidoptera species and other insects. However, B. thuringiensis preparations have proved to be of only limited value in combating insects such as those of the genera Spodoptera and Plutella, as well as various other lepidopteran pests. This toxicity of B. thuringiensis preparations against only certain pest species, or differential toxicity, is believed to be due to the expression of only certain endotoxin genes in any given B. thuringiensis variant, each toxin contributing in an unpredictable manner to the overall toxicity profile.
Numerous researchers have attempted to identify B. thuringiensis strains which have a broader or different spectrum of pesticidal activity, or to manipulate the B. thuringiensis genome to promote the expression of particular .delta.-endotoxins. Efforts directed to screening individual isolates for toxicity have led to the isolation of some previously unknown strains such as B. thuringiensis var. tenebrionis, dislosed by Krieg et al. in U.S. Pat. No. 4,766,203, issued Aug. 23, 1988, which strain is reportedly effective for combating Coleoptera. The use of conventional screening procedures to identify strains with new pesticidal activity, however, is time- and labor-intensive given the innumerable variants of B. thuringiensis that occur in nature.
Another approach has been to clone genes coding for B. thuringiensis .delta.-endotoxins and to re-arrange the B. thuringiensis genome in a beneficial way, or to introduce the cloned genes into new microbial hosts for expression. Herrnstadt et at., in U.S. Pat. No. 4,771,131, issued Sep. 13, 1988, describe the cloning of an M-7 toxin gene said to be suitable for expression in other microbes such as Pseudomonas and to thereby confer the ability to control beetles of the order Coleoptera. These methods, however, suffer from the drawback that the resulting organisms are subjected to increased regulatory scrutiny relative to organisms which occur naturally.
There is, therefore, a continued need for the identification of B. thuringiensis strains which display a broader or different spectrum of pesticidal activity. Ideally, such strains would be identified from among naturally occurring variants without the use of random screening methods.